We develop an equivariant version of Seiberg-Witten-Floer cohomology for finite group actions on rational homology 3-spheres. Our construction is based on an equivariant version of the Seiberg-Witten-Floer stable homotopy type, as constructed by Manolescu. We use these equivariant cohomology groups to define a series of d-invariants d G,c (Y, s) which are indexed by the group cohomology of G. These invariants satisfy a Frøyshov-type inequality under equivariant cobordisms. Lastly we consider a variety of applications of these d-invariants: concordance invariants of knots via branched covers, obstructions to extending group actions over bounding 4-manifolds, Nielsen realisation problems for 4-manifolds with boundary and obstructions to equivariant embeddings of 3-manifolds in 4-manifolds.
DAVID BARAGLIA AND PEDRAM HEKMATIHence we obtain a canonical isomorphismThis shows that the Seiberg-Witten-Floer cohomology HSW * (Y, s) does not depend on the choice of metric g. By working in an appropriately defined S 1 -equivariant Spanier-Whitehead category in which suspension by fractional amounts of C is allowed, Manolescu defined the Seiberg-Witten-Floer homotopy type of (Y, s) to beThis is independent of the choice of g by (1.1) and (1.2). Now suppose that a finite group G acts on Y by orientation preserving diffeomorphisms which preserve the isomorphism class of s. Let g be a G-invariant metric on Y . Lifting the action of G to the associated spinor bundle determines an S 1 extensionManolescu's construction of the stable homotopy type SW F (Y, s, g) can be carried out G s -equivariantly, so that SW F (Y, s, g) may be promoted to a G sequivariant stable homotopy type. This is analogous to the construction in [42] of the P in(2)-equivariant Seiberg-Witten-Floer stable homotopy type of (Y, s) where s is a spin-structure on Y . The main difference is that in our construction, the additional symmetries that comprise the group G s come from symmetries of Y rather than internal symmetries of the Seiberg-Witten equations.We define the G-equivariant Seiberg-Witten-Floer cohomology of (Y, s) to beThe right hand side is independent of the choice of metric g by much the same argument as in the S 1 -equivariant case. We make some remarks concerning this construction.(1) In this paper we have chosen to work throughout with cohomology instead of homology. This is simply a matter of preference and we could just as well work with Seiberg-Witten-Floer homology groups. (2) Instead of Borel equivariant cohomology, we could take co-Borel cohomology or Tate cohomology, which correspond to the different versions of Heegaard-Floer cohomology, [39, Corollary 1.2.4]. (3) In a similar fashion we can also define the G-equivariant Seiberg-Witten-Floer K-theory KSW * G (Y, s) = K * +2n(Y,s,g) Gs (SW F (Y, s, g)).More generally we could use any generalised equivariant cohomology theory in which the Thom isomorphism holds. (4) We have not attempted to construct a metric independent G s -equivariant stable homotopy type. To do this one would need to sp...